The Bing algorithm is not publicly available and because of that there is a lot of speculation about what matters more for Bing (in comparison to #Google) .
Here are some factors, which are ( believed to be) of importance when Bing optimization is concerned:
#Backlinks are of less importance. If you compare the first 10 results in Bing and Google, it is noticeable that the top rankers in Bing have less backlinks than those in Google. It is unclear if nofollow matters with Bing.
Inbound anchor text matters more. The quantity of quality inbound #links might be of less importance for Bing but the anchor text certainly matters more.
Onpage factors matter more in Bing than with Google. This is one of the most controversial points, but many #SEO experts think that onpage factors matter more with Bing than with Google.
Bing pays more attention to the authority of the site. If this is true, this is bad news for bloggers and small sites because it means that search results are distorted in favor of older sites and/or sites of authoritative organizations. Age of domain is also very important with Bing.
PR matters less. When you perform a search for a competitive keyword and you see a couple of PR2 or even PR1 sites among the top 10 results. On Google this is hardly possible but on Bing it looks quite normal.
Fresh content matters less. Bing looks a bit conservative but it seems that fresh content is not so vital as with Google. This is related to the age of domain specifics and as a result you will see ancient pages rank high (but these ancient pages are relevant to the search query).
Bing is more Flash-friendly. It looks like Bing is more Flash-friendly, which is good news to all sites where Flash is (still) heavily employed.
It may be too early to say which factors are of primary importance with Bing, but the fact that their search results are relevant means that their algorithm is really precise. As a result, the content is authentic, as there are less known SEO gimmicks. We’ll see if this will stay so in the future, when SEO experts learn how to optimize for Bing as well!